The conventional extraction process, the oldest known, consists in grinding the canes with an impact disk mill in order to open the cells, and then passing the preparation obtained through cane mills consisting of fluted rollers which exert a high pressure of 50 to 100 bars between them. The outfit is relatively expensive and consumes a large amount of energy for driving these rollers. Additionally, for producing high yields, the outfit needs to be of a considerable size.
The use of the diffusion process for the extraction of beet sugar has cost the manufacturers to seek the adaptation thereof for cane sugar. In such a diffusion process, ground canes are suspended in water in order to carry out the extraction by diffusion, maceration and leaching. The diffusion is carried out in large equipment according to the conventional countercurrent exhaustion process, which leads to significant extraction times and especially to large tank volumes, therefore to expensive and bulky outfits, with additionally difficulties of operation, especially due to fermentation problems.
The conventional process was later re-adopted to improve it and to lead to the re-soaking process. This process essentially consists in irrigating the prepared cane with extracted juice and crushing it again in a mill following the first, the process being carried out by means of several successive re-soakings and several mills, in order to improve the extraction yield. The result is that the efficiency of the whole equipment is improved. Despite this improvement, the complete outfit remains expensive, bulky and high energy consuming, as against the increase in the extraction yield.
This last process has been the subject of many improvements, some of which have brought to light the harmful role played by the absorption of air by cells which expand elastically immediately after crushing. In fact, this air prevents an effective soaking by the liquid and unnecessarily consumes energy by its compressibility. In order to avoid this, some of the processes proposed provide for the de-aeration of the mixtures, or for the irrigation of the bagasse immediately after it leaves the mill, but without ever achieving the complete elimination of this presence of air.